I. Field of the Invention
The present invention relates to wireless communication systems. More particularly, the present invention relates to a novel and improved method for enabling a mobile station to execute a handoff between an asynchronous base station and a synchronous base station in a CDMA wireless communication system.
II. Description of the Related Art
FIG. 1 is an exemplifying embodiment of a terrestrial wireless communication system 10. FIG. 1 shows the three remote units 12A, 12B and 12C and two base stations 14. In reality, typical wireless communication systems may have many more remote units and base stations. In FIG. 1, the remote unit 12A is shown as a mobile telephone unit installed in a car. FIG. 1 also shows a portable computer remote unit 12B and the fixed location remote unit 12C such as might be found in a wireless local loop or meter reading system. In the most general embodiment, remote units may be any type of communication unit. For example, the remote units can be hand-held personal communication system units, portable data units such as personal data assistants, or fixed location data units such as meter reading equipment. FIG. 1 shows a forward link signal 18 from the base stations 14 to the remote units 12 and a reverse link signal 20 from the remote units 12 to the base stations 14.
An industry standard for a wireless system using code division multiple access (CDMA) is set forth in the TIA/EIA Interim Standard entitled “Mobile Station—Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System”, TIA/EIA/IS-95, and its progeny (collectively referred to here in as IS-95), the contents of which are also incorporated herein by reference. More information concerning a code division multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, assigned to the assignee of the present invention and incorporated in its entirety herein by this reference.
Third-generation CDMA wireless communications systems have also been proposed. The cdma2000 ITU-R Radio Transmission Technology (RTT) Candidate Submission proposal forwarded by the Telecommunications Industry Association (TIA) to the International Telecommunication Union (ITU) for consideration for the IMT-2000 CDMA standard is an example of such a third-generation wireless communication system. The standard for cdma2000 is given in draft versions of IS-2000 being generated by TR45 of the TIA. The cdma2000 proposal is compatible with IS-95 systems in many ways. For example, in both the cdma2000 and IS-95 systems, each base station time-synchronizes its operation with other base stations in the system. Typically, the base stations synchronize operation to a universal time reference such as Global Positioning Satellites (GPS) signaling; however, other mechanisms can be used. Based upon the synchronizing time reference, each base station in a given geographical area is assigned a sequence offset of a common pseudo noise (PN) pilot sequence. For example, according to IS-95, a PN sequence having 215 chips and repeating every 26.66 milliseconds (ms) is transmitted by each base station in the system at one of 512 PN sequence offsets as a pilot signal. The base stations continually transmit the pilot signal which can be used by the remote units to identify the base stations as well as for other functions.
Base station time-synchronization as provided in the cdma2000 and IS-95 systems has many advantages with respect to system acquisition and handoff completion time. Synchronized base stations and time-shifted common pilot signals as discussed above permit a fast one-step correlation for system acquisition and detection of neighboring base stations. Once the mobile station has acquired one base station, it can determine system time which is the same for all neighboring synchronous base stations. In this case, there is no need to adjust the timing of each individual mobile station during a handoff between synchronous base stations. Additionally, the mobile station does not need to decode any signal from the new base station in order to obtain rough timing information prior to handing off.
Another recently-proposed 3G communication system is referred to as W-CDMA. One example of a W-CDMA system is described in the ETSI Terrestrial Radio Access (UTRA) International Telecommunications Union (ITU) Radio Transmission Technology (RTT) Candidate Submission forwarded by ETSI to the ITU for consideration for the IMT-2000 CDMA standard. The base stations in a W-CDMA system operate asynchronously. That is, the W-CDMA base stations do not all share a common universal time reference. Different base stations are not time-aligned. As a result, W-CDMA base stations employ a 3-step acquisition procedure with multiple parallel correlations in each step. In the W-CDMA system, each base station transmits a “synchronization” channel that comprises two sub-channels. The first of the two sub-channels, the primary synchronization channel, uses a primary synchronization code, cp, that is common to all base stations. The second of the two sub-channels, the secondary synchronization channel, uses a cyclic set of secondary synchronization codes, cs, that are not shared by other base stations that are not in the same code group. The mobile station in a W-CDMA system can acquire the synchronization channel of one or more base stations by searching for the primary synchronization code, cp of the primary synchronization channel, and then using the timing information derived from the primary synchronization channel to process the secondary synchronization channel.
Recently, a combined CDMA IMT-2000 standard has been proposed in which cdma2000-compliant equipment and W-CDMA-compliant equipment may be optionally supported by any manufacturer. Thus, it is expected that synchronous base stations of a cdma2000-compliant system will be geographically located near asynchronous base stations of a W-CDMA-compliant system. This creates a need to be able to handoff a mobile station that supports both cdma2000 and W-CDMA operation between the asynchronous base stations of a W-CDMA system and the synchronous base stations of a cdma2000 system, and vice versa.
U.S. Pat. No. 5,267,261 entitled “MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM,” which is assigned to the assignee of the present invention and which is incorporated herein, discloses a method and system for providing communication with the remote unit through more than one base station during the handoff process. Further information concerning handoff is disclosed in U.S. Pat. No. 5,101,501, entitled “METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM”, U.S. Pat. No. 5,640,414, entitled “MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM”, and U.S. Pat. No. 5,625,876 entitled “METHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATION,” each of which is assigned to the assignee of the present invention and incorporated in its entirety herein by this reference. The subject matter of U.S. Pat. No. 5,625,876 concerns so-called “softer handoff.” For the purposes of this document, the term “soft handoff” is intended to include both “soft handoff” and “softer handoff.” U.S. Pat. No. 6,456,606, entitled “HANDOFF CONTROL IN AN ASYNCHRONOUS CDMA SYSTEM”, assigned to the assignee of the present invention and incorporated in its entirety herein by reference, also provides additional information on handoff procedures involving both synchronous and asynchronous base stations.
Each base station is associated with a set of neighboring base stations surrounding the base station. Due to the physical proximity of the coverage areas of the neighboring base stations to the coverage area of the active base station, the remote units which are communicating with the active base station are more likely to handoff to one of the neighboring base stations than to other base stations in the system. In the IS-95 and cdma2000 systems, the base station identifies the neighboring base stations to the remote units with which it has established communication using a neighbor list identification message. The neighbor list identification message identifies a neighboring base station according to the PN sequence offset at which it transmits the pilot signal. In the IS-95 and cdma2000 systems, there is a one-to-one correspondence in a given geographical area between a base station and a PN sequence offset. In other words, two base stations operating in the same geographical area do not both use the same PN sequence offset. Thus, a base station in the IS-95 or cdma2000 system can be uniquely identified in a geographical region by its PN sequence offset.
The remote unit uses the neighbor list to limit the space over which it searches for handoff candidates. Because the searching process is so resource intensive, it is advantageous to avoid performing a search over the entire set of possible PN sequence offsets. By using the neighbor list, the remote unit can concentrate its resources on those PN sequence offsets which are most likely to correspond to useful signal paths.
A typical IS-95 or cdma2000 neighbor acquisition operation is practical so long as each base station's timing remains synchronous with respect to the others. However, in some systems such as W-CDMA, advantages are achieved by decoupling operation of the system from a synchronizing reference. For example, in a system which is deployed underground, such as in a subway system, it can be difficult to receive a universal time synchronization signal using GPS. Even where strong GPS signals are available, it is perceived as desirable in some political climates to decouple system operation from the U.S. Government GPS system. There may be other reasons for decoupling operation of the system from a synchronizing reference.
In a system where one or more of the base stations operate asynchronously with respect to other base stations in the system, the base stations cannot be readily distinguished from one another based merely upon a relative time offset (typically measured as a relative PN sequence offset) because a relative time offset between the base stations cannot be established without the use of a common time reference. Thus, when a remote unit is in communication with an asynchronous base station, and has not been recently in communication with a synchronous base station, the remote unit is unlikely to have system time information of the synchronous base stations to a sufficient accuracy.
For example, suppose a remote unit has been in the coverage area of an asynchronous base station and is moving into the coverage area of a synchronous base station. Further suppose that the remote unit is able to detect the pilot signals of two different synchronous base stations by determining their relative PN sequence offsets. Unless the remote unit already knows system time of the synchronous base stations to a sufficient accuracy, the remote unit will be unable to determine which pilot signal is being transmitted by which base station. In other words, although the remote unit is able to distinguish that there are two different synchronous base stations due to their different relative PN sequence offsets, the remote unit is unable to determine the identity of either synchronous base station based on their pilot signals alone because the remote station does not have an absolute time reference with which to compare the two PN sequence offsets.
In a conventional IS-95 or cdma2000 system, once the forward pilot channel is acquired, the remote unit can then demodulate the forward Sync channel. This is possible because the forward sync channel timing is such that its frame boundary is always aligned to the beginning of the PN sequence of the forward pilot channel. In other words, the forward sync channel frame boundary is always offset from system time by the same number of PN chips as the PN sequence offset of the corresponding forward pilot channel. The forward sync channel carries a sync channel message which includes overhead information such as system identification, system time, the base station's PN sequence offset, and several other items of useful information. After demodulating the sync channel message, the remote unit adjusts its internal timing according to the PN offset and system time sent in the sync channel message as described in IS-95.
Because the conventional sync channel is transmitted at a low data rate (for example, 1200 bps in IS-95), and the sync channel message contains a large amount of overhead information that must be demodulated on a frame-by-frame basis, it may be on the order of 800 milliseconds before the remote unit is able to determine the system identity of the transmitting base station via the sync channel message. This delay can undesirably affect the timing of a handoff from the asynchronous base station to the synchronous base station, particularly in a fading environment. In some instances, the delay associated with the remote unit having to determine the system identification of the target synchronous base station(s) by demodulating a conventional sync channel message would be unacceptably long, causing degradation or even dropping of a call in progress.
Thus, there is a need for an improved method for facilitating handoff between asynchronous and synchronous base stations that avoids the undesirable delays associated with demodulating a conventional sync channel message.